Please visit Canon's site and download the latest driver, below is a link to their driver site for the Pixma MG6821 - please make sure to choose Mac OS X Yosemite in the Operating System drop down:
There are two drivers that were released after your purchase, hopefully one of those will be what you need:
MG6800 series CUPS Printer Driver Ver. 18.104.22.168 (Mac)
ICA Driver Ver.4.1.3 (Mac)
I apologize that the information I provided was insufficient. Since you've had the printer for only 3 months, have you tried contacting Canon directly? Most manufacturers provide a 1 year warranty, and hopefully they would offer some technical advice during that period also. I will opt out of this question also, in case another professional can answer.